1. Field of the Invention
The present invention relates to an eye examining instrument, and more particularly to a system for focusing the instrument on the fundus of an eye to be examined. The present invention is also directed to improvements in the eye-examining apparatus already proposed by our prior applications, U.S. application Ser. No. 109,275 (abandoned in favor of continuation U.S. application Ser. No. 346,870, filed Feb. 2, 1982), and Ser. No. 180,103.
2. Description of the Prior Arts
In a conventional eye-fundus camera, focusing on the eye has been achieved by adapting the sight of the eye to cross-hairs in a finder and by adjusting the photographing lens in such a manner that the eye-fundus can be clearly viewed in this state. However this focusing method inevitably involves fluctuation during an examination and is difficult to conduct rapidly.
U.S. Pat. No. 3,925,793 (German Pat. No. 2,415,319) and U.S. patent application Ser. No. 945,845 now U.S. Pat. No. 4,283,124 relate to a focusing method by projecting plural marks on the eye-fundus and aligning the marks under observation. Although rapid focusing has been rendered possible by these methods, there still remains a strong demand for automatic focusing of the camera, as the setting of an eye-fundus camera simultaneously requires alignment of the eye axis with the optical axis of the objective lens, distance adjustment between the cornea and the objective lens and focus adjustment. The operator has to constantly pay attention to these three factors as the above-mentioned alignment and distance are easily affected by small movements of the subject to be examined while the focusing is affected by a change in the sight of the eye to be examined. For this reason automatic focusing, if realized, will significantly alleviate the load on the operator and contribute to the probability of obtaining photographs of improved image quantity.
As a pioneer invention for automatic focusing of the eye-fundus camera to the eye ground there is U.S. Pat. No. 3,614,214, in which a dichroic mirror reflecting infrared light but transmitting visible light is provided in front of an ordinary eye-fundus camera in an oblique position to deflect a detecting beam from an automatic optometer toward the eye to be examined and to again deflect reflected beam from the eye to the optometer, whereby the focusing lens of the eye-fundus camera being adjusted by the output of the optometer.
As readily seen from the above, the known eye-fundus cameras need refocusing even after the camera has once been focused on the fundus if the subject moves even very slightly or any change occurs in the sight of the subject eye. In this case there exists some time lag as a matter of course. Therefore it is desirable to display the state of focusing with means such as a focus mark. If such a focus mark display is realized, it will enable the operator or examiner easily to release the shutter because he can confirm that the apparatus is in-focus by viewing the focus mark. Furthermore, the operator can release the shutter at the proper time which in turn serves to improve the image quality. In addition, such focus mark display will make it possible to carry out manual focusing rapidly.
However, the use of a focus mark in an eye-fundus camera involves problems. The focus detecting light reflected upon the fundus and guided to the view-finder enters a photodetector and constitutes noise by which the accuracy of detection is substantially reduced.